An air duct used for blowing air through a ceiling segment for air conditioning of an automobile in which the duct is comprised of a base material showing a superior leakage prohibiting characteristic, a high extension rate at the time of molding operation and facilitating a deep grave molding or forming operation. An automobile air duct is manufactured by applying a molding or forming member having three-layer complex plastic sheet materials having formed of an expanded polypropylene sheet and mica-containing polypropylene sheets stacked on both sides thereof.